In recent years, apparatus having electromagnetic wave generation sources, not to speak of radio transmitters and high-frequency therapeutic apparatus, have increased in utilization and the malfunction of a pyroelectric infrared sensor which is caused by electromagnetic wave noise generated from those apparatus has been taken up as a tough problem. Specifically, the pyroelectric infrared sensor, having a very weak output, is commonly used by amplifying the output through a high gain amplifier and consequently is liable to be affected by the noise. In addition to this, a problem has been encountered that, since input impedance to the amplifier turns extremely high, a part provided on the input side thereof behaves easily as an antenna and, as a result, the malfunction caused by the electromagnetic wave noise is liable to be generated.
FIG. 6 is a circuit diagram showing a conventional pyroelectric infrared sensor and an amplification circuit connected thereto. Reference numeral 1 denotes a dual type pyroelectric element, 2 an FET for impedance transposition, whose gate G is connected to one electrode of the pyroelectric element 1, and 3 a leakage resistance connected between the gate G of the FET 2 and a ground E connected to the other electrode of the pyroelectric element 1 and preventing the gate of the FET 2 from being saturated in the case where an excessive input is caused. These components are housed in the same package 4 to construct the pyroelectric infrared sensor. Reference symbols l.sub.1, l.sub.2, l.sub.3 denote output lead-in wires connected to a drain D and a source S of the FET 2 and the ground E, respectively. Further, reference numeral 5 denotes an output load resistance for fetching an output signal from the source S, connected between the output lead-in wires l.sub.2 and l.sub.3, 6 a noise out capacitor for high-frequency noise leakage, likewise connected between the output lead-in wires l.sub.2 and l.sub.3, and 7 a high gain amplifier connected to the output lead-in wire 2. These components configure the amplification circuit.
In the conventional pyroelectric infrared sensor constructed as described above, although attempts have been made to cut the electromagnetic wave noise which is high-frequency noise, by means of the capacitor 6 provided outside, there has been a problem that the noise cannot completely be cut only by the capacitor 6. Specifically, in FIG. 6, when a noise voltage inputted to the connection between the output lead-in wire l.sub.2 and the capacitor 6 is represented by Nv, the impedance of the capacitor 6 by Z, and the resistance of the output lead-in wire l.sub.2 by R, a noise voltage Nv' outputted from the connection is expressed by ##EQU1## Here, although, if the frequency of the noise voltage Nv turns high, the impedance is reduced, Nv'.apprxeq.Nv due to R .apprxeq.O and the noise is not completely cut.
Further, the invasion of the electromagnetic wave noise is made through routes such as the lead-in wires, l.sub.1, l.sub.2 and a window 4a'formed of silicon, and attempts have also been made to prevent the noise invasion for each route. These attempts are such that, for example, a filter for preventing the noise invasion is provided for each lead-in wire and a netted member comprising a good electric conductor is attached to the window 4a' formed of silicon. Such manners, however, make it difficult to bring about an effect sufficient for preventing the malfunction on all the routes of the noise invasion and may cause problems that the increase of the number of parts incurs cost increase, large-sizing of apparatus, and reduction of sensitivity.
Also, even if a filter sufficient to absorb the electromagnetic wave noise can be constructed, it may turn to an antenna of the electromagnetic wave in the case where the filter is positioned on the outside of the package 4, that is, where it is in a non-shield state, so that a problem arises that a noise attenuation effect is small. In addition, although attempts have been made to incorporate the sensor and the amplification circuit in the same package and thereby to eliminate the invasion of the electromagnetic wave noise, it is difficult to make a complete shield because the package needs an incident window for infrared rays. As a result, the use of a large-sized package brings about problems of the large-sizing of the sensor and the increase of manufacturing costs.
The present invention, therefore, is to provide a pyroelecteic infrared sensor which can prevent any malfunction resulting from the electromagnetic wave noise without causing the problems of the large-sizing of the sensor and the increase of manufacturing costs.